The invention claims priority of British Patent Application No. 9801138.0, filed Jan. 20, 1998, titled xe2x80x9cTime-Temperature Indicator Devices,xe2x80x9d by Michael J. Simons.
The invention relates to time-temperature indicator devices.
Time-temperature indicator devices provide a way of indicating a cumulative time-temperature exposure. A device may be capable of indicating whether a commodity has been exposed to a temperature greater than a predetermined temperature for a period of time. For example, a device might indicate exposure to an excessive temperature for an excessive time or exposure to an adequate temperature for an adequate time.
Various types of time-temperature indicator devices have been disclosed.
U.S. Pat. No. 4,212,153 discloses a time indicator device in which molecular migration of a dye or other agent from one surface of a polymeric indicator layer to the other causes a visible change after a defined time. The indicators exemplified make use of plasticized polyvinyl chloride as the polymeric material. Examples are shown where the rate of appearance of dye color is accelerated by higher temperatures.
U.S. Pat. No. 5,057,434 describes an integrated time-temperature indicator device comprising a system of printable multilayer compatible compositions including (i) a layer of a first composition (a) which develops a first color change as a result of a cumulative time-temperature exposure; (ii) a composition (b) that is capable of melting at a predetermined temperature; and (iii) a composition (c) capable of producing a second color change only as a result of the melting of composition (b) and wherein said second color change is combined with the first color change to yield an integrated color change result.
U.S. Pat. No. 5,476,792 discloses a time-temperature indicator device for recording the duration of time over which a temperature has been established above a predetermined temperature. The device includes a dye-compatible polymer composition having a softening point above the predetermined temperature and a polymer-compatible dye composition in contact with the polymer composition and present in a quantity sufficient for diffusion through the polymer composition whenever it is above the predetermined temperature. The quantity of dye is selected so that the extent of diffusion corresponds to the duration of exposure above the predetermined temperature relative to the total time required for substantially complete diffusion. The devices exemplified are suitable for providing a heat history for medical instruments subjected to high temperature sterilisation.
The present invention is directed to overcoming one or more of the problems set forth above. Briefly summarised, according to one aspect of the present invention, the invention provides a time-temperature indicator device comprising a polymeric layer having a first and second surface and a dye composition adhered to said first surface comprising a dye which diffuses into the polymeric layer as a result of a cumulative time-temperature exposure wherein the polymeric layer is formed from a natural or synthetic rubber polymer or copolymer.
In another aspect, the invention provides a method of forming a time-temperature indicator device comprising coating a layer of an aqueous emulsion of a natural or synthetic rubber polymer or copolymer on a support, drying the layer and applying to the surface of the layer a dye composition comprising a dye which diffuses into the polymeric layer as a result of a cumulative time-temperature exposure.
A method of monitoring the cumulative time-temperature exposure of a product comprises associating a time-temperature indicator device according to the invention with the product and observing the degree of dye diffusion over a period of time.
These and other aspects, objects, features, and advantages of the present invention will be more clearly understood and appreciated from a review of the following detailed description of the preferred embodiments and appended claims, and by reference to the accompanying drawings.
Alternative time-temperature indicator devices are desired which are simple to manufacture.
There is a need for devices which can be activated at the point of use without need for appropriate storage before use.
Devices free of plasticizer may be required to avoid the contamination which can result from the leaching of plasticizer.
The present invention has the following advantages:
The simplicity of the system makes it inherently easy and low cost to produce;
The activation of the device at the point of use avoids the need for appropriate storage before use; and
Devices free of plasticizer can be made.